Backport of r60190:

- Fix Issue #1703448: A joined thread could show up in the
  threading.enumerate() list after the join() for a brief period until
  it actually exited.

Lib/threading.py

@@ -524,11 +524,17 @@ class Thread(_Verbose):
                 if __debug__:
                     self._note("%s.__bootstrap(): normal return", self)
         finally:
-            self.__stop()
+            _active_limbo_lock.acquire()
             try:
-                self.__delete()
-            except:
-                pass
+                self.__stop()
+                try:
+                    # We don't call self.__delete() because it also
+                    # grabs _active_limbo_lock.
+                    del _active[_get_ident()]
+                except:
+                    pass
+            finally:
+                _active_limbo_lock.release()
 
     def __stop(self):
         self.__block.acquire()